1. Field of the Invention
The present invention relates to an image processing apparatus operable in plural copying modes, and more particularly, to an image processing apparatus operable in plural copying modes such as an electrophotographic copying machine comprising analogue and digital copying modes.
2. Description of Related Art
Conventionally, as electrophotographic copying machines, there have been provided an analogue copying machine for forming a latent image on a photoconductor and developing the latent image into a visible toner image so as to print an so-called analogue image on a sheet of copying paper, and a digital copying machine for printing dot images or so-called digital image on a sheet of copying paper according to digital image data.
In the digital copying machine, after an image of a document (referred to as a document image hereinafter) is read and converted into analogue electric signals by an image sensor, the analogue electric signals are converted into digital image data by an analogue to digital converter. Further, an exposure head such as an LED head forms dot images on a photoconductive drum according to the digital image data so as to form an electrostatic latent image thereon. Thereafter, in the manner well known to those skilled in the art, the electrostatic latent image is developed into a visible toner image with toner, and the toner image is transferred onto a sheet of copying paper.
The digital copying machine has such advantages that not only an editing operation such as a trimming operation can be performed, since digital image data can be processed by a digital electric circuit, but also digital image data can be stored in a storage unit and can be sent to an external unit.
On the other hand, the digital copying machine has such disadvantages that it is difficult not only to reproduce a half tone image but also to obtain a resolution higher than a predetermined resolution. In order to reproduce a half tone image, the dither matrix method is used generally. However, in this method, the resolution of the reproduced half tone image becomes low, resulting in degradation in the quality of the reproduced binary image such as character image or the like. Further, in order to heighten the resolution of the reproduced image, it is necessary to process a large amount of data for representing an image. Therefore, the digital copying machine requires not only a storage unit having a large memory capacity but also a longer processing time. Furthermore, when an image which has copied by a digital copying machine is recopied by a digital copying machine, in many cases, degradation in the quality of the recopied image is caused due to the moire phenomenon. Furthermore, when a half tone image which has been copied by a digital copying machine is recopied by a digital copying machine, there is caused remarkable degradation in the quality of the recopied image which is produced by a so-called generation copying operation. Therefore, it is necessary to utilize either an analogue copying machine or a digital copying machine depending on the use for copying an image and/or a document.
In order to solve the aforementioned problems of the conventional digital copying machine, there is proposed a method for reproducing both a binary image and a half tone image by processing image data so as to obtain a predetermined resolution, a method for performing the generation copying operation by reducing the moire phenomenon using a filtering method, and a method for reading an image with use of an image reader having a higher resolution and for writing the read image in a high density so as to reproduce an image in a higher quality on a sheet of copying paper. However, in all the methods, the composition of the copying machine becomes complicated, resulting in increase in the manufacturing cost and decrease in the processing speed thereof.
On the other hand, documents which are copied in offices are binary information such as characters, numerals, line drawings, and also it is necessary to often produce a hard copy of image data of an half tone image. Therefore, the analogue copying machine has higher performance in works in offices than the digital copying machine, generally.
In order to perform various kinds of copying operation, there has been proposed a copying machine of composite type comprising an analogue copying system and a digital copying system. In the copying machine of this type, the analogue and digital copying systems utilize a common electrophotographic printing section. However, the analogue copying system utilizes an exposure optical system for projecting light reflected from a document onto a photoconductor. On the other hand, the digital copying system utilizes an image read section using a CCD image sensor or the like, and an image write section such as laser printer or the like. Either the analogue copying system or the digital copying system of the copying machine of composite type can be used depending on the use so as to make the best use of the features of each copying system. For example, when a copy which has been produced by the digital copying system is recopied, degradation in the quality of the image can be reduced by reproducing the copy using the analogue copying system. Furthermore, an analogue half tone image and a digital binary image can be composed by the copying machine of composite type.
In a copying machine of composite type proposed in the Japanese patent laid open publication (JP-A) No. 55-123270/1980, there are performed an image write process for writing an image on a photoconductor in the analogue copying mode and an image read process using a CCD image sensor in the digital copying mode, wherein the image write process and the image read process are performed using separate optical systems, respectively. It is to be noted that, in this copying machine of composite type, a digital image write process can be performed using optical fiber tubes. Therefore, this copying machine of composite type has such a disadvantage that the composition thereof becomes complicated.
Furthermore, in a two-color copying machine proposed in the Japanese patent laid open publication (JP-A) No. 56-83757/1981, an analogue copying operation is performed in a positive exposure process, and a digital image write operation is performed in a negative exposure process using a laser scan optical system and optical fiber tubes. This copying machine of this type has such disadvantages that the composition thereof becomes large and the manufacturing cost thereof becomes high since the laser scan optical system is used in the digital image write operation. It is to be noted that the copying machine of this type can not be solely used as a digital copying machine since it is necessary to use another optical system which is arranged in another machine in order to perform an image read operation.
When an LED head having a small configuration is used as an image write head used in the digital copying operation, an image write optical system comprising the LED head becomes smaller than a laser scan optical system, resulting in that the copying machine can be miniaturized. Therefore, a copying machine of composite type for utilizing not only the functions and the composition of the analogue copying machine but also an LED head as an image write head with combining the peripheral circuits therewith can be constituted without a complicated composition.
However, when the power is supplied to the LED head in order to light the LEDs thereof, the LEDs radiate heat. Therefore, if the LEDs are turned on for a long time interval, the life of each LED is shortened. By the way, the logic of digital image data to be used for lighting the LEDs can be inverted easily by an inverter. Therefore, the LEDs can be lighted so as to write not only an image portion of a document but also a non-image portion thereof on a photoconductor. However, since an area of an image portion of a document is generally smaller than that of a non-image portion thereof, the LEDs are preferably lighted so as to form the image portion of the document on a photoconductor.
However, in the analogue copying operation, light of non-image portion of a document reflected from the document is projected onto the photoconductor so as to form the image of the non-image portion thereon. Therefore, the LEDs can not be lighted so as to form the image portion thereof on the photoconductor in the digital copying operation by only combining the LED head with the analogue copying machine.
In a two-color copying machine proposed in the Japanese patent laid open publications, for example, (JP-A) Nos. 54-104834/1979 and 56-83757/1981, a two-color copying operation is performed in one copying cycle with use of two kinds of toners having different colors and different polarities of the electric potentials. In the copying machine of this type, latent images of respective colors are formed on a photoconductor with use of separate image write systems such as a projection optical system for projecting a document image on the photoconductor and a laser scan optical system or optical fiber tubes etc. and two pairs of multi stylus heads, so as to have different polarities of the electric potentials, resulting in two-color toner image.